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  • Antibody Conjugate Therapeutics: Challenges and Potential

    Beverly A. Teicher1 and Ravi V.J. Chari2

    Abstract Antibody conjugates are a diverse class of therapeutics consisting of a cytotoxic agent linked covalently

    to an antibody or antibody fragment directed toward a specific cell surface target expressed by tumor cells.

    The notion that antibodies directed toward targets on the surface of malignant cells could be used for drug

    delivery is not new. The history of antibody conjugates is marked by hurdles that have been identified and

    overcome. Early conjugates usedmouse antibodies; cytotoxic agents thatwere immunogenic (proteins), too

    toxic, or not sufficiently potent; and linkers that were not sufficiently stable in circulation. Investigators have

    explored 4 main avenues using antibodies to target cytotoxic agents to malignant cells: antibody-protein

    toxin (or antibody fragment–protein toxin fusion) conjugates, antibody-chelated radionuclide conjugates,

    antibody–small-molecule drug conjugates, and antibody-enzyme conjugates administered along with

    small-molecule prodrugs that require metabolism by the conjugated enzyme to release the activated

    species. Only antibody-radionuclide conjugates and antibody-drug conjugates have reached the regulatory

    approval stage, and nearly 20 antibody conjugates are currently in clinical trials. The time may have come

    for this technology to become a major contributor to improving treatment for cancer patients. Clin Cancer

    Res; 17(20); 6389–97. �2011 AACR.

    Introduction

    The challenges posed by the discovery of therapeuti- cally effective antibody conjugates are as formidable as those encountered in the discovery and development of small-molecule drugs. Over the past 20 years, many cell surface proteins that have selective aberrant expression on malignant cells or are aberrantly highly expressed on the surface of malignant cells have been identified. In some cases, specific antibodies that bind tightly to such proteins were developed. Unfortunately, not infrequently, exposing the tumor cells in culture or treating human tumor xenograft-bearing mice with these antibodies did not alter tumor growth. Antibody con- jugates provide an opportunity to make use of antibodies that are specific to cell surface proteins and thus offer some important advantages over current therapeutics, such as improved target specificity and potency. How- ever, this approach has some limitations, notably for the treatment of solid tumors, such as the difficulty of delivering a macromolecule to solid tumors, hetero- geneity of antigen expression on the tumor surface, and

    expression of the antigen by normal tissues (Table 1). Hematological malignancies composed of leukemia or lymphoma cells may be more amenable to treatment with antibody conjugates in view of the ready accessi- bility of these cells. Typically, antigen expression is specific and homogeneous, although the actual number of antigens on the cell surface may be lower than that found on solid tumors. This issue of CCR Focus offers insight into some of the key criteria to consider in the development of antibody conjugates.

    The notion that antibodies directed toward targets on the surface of malignant cells could be used for drug, radionuclide, or cytotoxic protein delivery is not new. In the late 1980s, after great efforts, a few mouse antibodies that went into clinical trials were found to be inactive and also rapidly neutralized by the immune system of patients. Subsequently, the idea of using these same antibodies to deliver powerful tumoricidal agents in a single or limited number of doses emerged. Over the next several years, investigators explored 4 main avenues using antibodies to target cytotoxic species to malignant cells: antibody-protein toxin conjugates (or antibody-protein toxin fusion proteins), antibody-radionuclide conjugate, antibody–small-molecule drug conjugates, and antibody- enzyme conjugates administered along with small-mole- cule prodrugs [also called antibody-directed enzyme prodrug therapy (ADEPT)], which require metabolism by the conjugated enzyme to release the active drug (1–5). ADEPT will not be further discussed in this CCR Focus because this technology has not advanced due to drawbacks, such as the immunogenicity of the bacterial enzyme component and the short half-life of the conjugates.

    Authors' Affiliations: 1Division of Cancer Treatment and Diagnosis, National Cancer Institute, National Institutes of Health, Bethesda, Mary- land; 2ImmunoGen, Inc., Waltham, Massachusetts

    Corresponding Author: Beverly A. Teicher, Molecular Pharmacology Branch, Developmental Therapeutics Program, National Cancer Institute, MSC 7458, Room 8156, 6130 Executive Blvd., Rockville, MD 20852. Phone: 301-594-1073; Fax: 301-480-4808; E-mail: Beverly.Teicher@nih.gov or teicherba@mail.nih.gov

    doi: 10.1158/1078-0432.CCR-11-1417

    �2011 American Association for Cancer Research.

    CCRFOCUS

    www.aacrjournals.org 6389

    on June 11, 2020. © 2011 American Association for Cancer Research. clincancerres.aacrjournals.org Downloaded from

    http://clincancerres.aacrjournals.org/

  • Antibody-Protein Toxin Conjugates (Immunotoxins)

    Thefirst tumoricidal agents tobe linked to antibodieswere potent, plant-derived protein toxins, such as gelonin, ricin, abrin, and pokeweed antiviral protein, and bacterial toxins such, as Pseudomonas exotoxin and Diphtheria toxin (6–8). Some of these immunotoxins were tested in the clinic with little success, and interest in this approach waned. The identified shortcomings included immunogenicity of the murine antibody and the protein toxin, rapid clearance from the blood stream, and systemic toxicity at low doses. In addition, these early immunotoxinswere composedof intact IgGs linked to full-length toxins by chemical couplingmeth- ods and thus were large in size, potentially limiting pene- tration into solid tumors, and chemically heterogeneous (9). The lessons learned during these explorations have led to improvements in the design of immunotoxins. The second- generation immunotoxinsweremadewith the use of recom- binant techniques whereby the DNA sequences encoding only the antigen-binding site of the antibody (the Fv portion engineered as a single chain) were fused to DNA sequences encoding the toxin, and thus were much smaller in size and homogeneous. In a further refinement applied to Pseudomo- nas exotoxin, a truncated form lacking the cell surface bind- ing domain was fused to the scFv portion of the antibody. Two different versions of anti-CD22-Pseudomonas exotoxin conjugate targeting B-cell malignancies are currently under clinical evaluation (10). The first version, called BL22 [RFB4- (dsFv)-PE38], showed significant activity in a phase II trial in patients with hairy cell leukemia (n¼ 36), with an overall response rate of 50%. Because the activity of BL22wasmuch

    lower in other B-cell malignancies [i.e., chronic lymphocytic leukemia, acute lymphoblastic leukemia (ALL), and non- Hodgkin’s lymphoma], an improved version of BL22, called moxetumomab pasudotox, with a higher binding affinity for CD22 and greater in vitro potency, was developed. In a phase I trial conducted in patients with hairy cell leukemia (n ¼ 32), moxetumomab pasudotox showed a slightly better complete response rate than its predecessor, BL22 (31% vs. 25%, respectively). Clinical trials in other hematological malignancies are ongoing (10).

    Although this class of immunotoxins may have mean- ingful activity in isolated disease settings, particularly in certain hematologic malignancies, the fundamental prob- lem of immunogenicity and fast clearance will continue to limit their therapeutic activity. In addition, the maximum tolerated dose achievable with such immunotoxins is very low (�0.05 mg/kg). The low dose coupled with fast clear- ance will likely limit localization to solid tumors: even for an intact IgG with a long half-life, the amount of antibody that gets to the tumor is

  • therapeutic activity in solid tumors. Despite the high affinity for CC49 antigen, the dose delivered to the tumor was only in the range of 0.2 to 6.7 Gy (16). Thus, the effectiveness of radioimmunotherapy may be limited to radiosensitive diseases such as lymphomas.

    Antibody-Drug Conjugates

    The concept of antibody-drug conjugates (ADC) evolved from the hope that targeted delivery with monoclonal antibodies would confer a degree of tumor selectivity to approved anticancer drugs and thus improve their thera- peutic index. Early ADCs were composed of tumor-specific murine monoclonal antibodies covalently linked to anti- cancer drugs, such as doxorubicin, vinblastine, and meth- otrexate. These early conjugates were evaluated in human clinical trials but had limited success due to immunogenic- ity, lack of potency, and insufficient selectivity for tumor versus normal tissue. The lessons learned from these early explorations led to improvements in essentially all aspects of antibody conjugate therapeutics and hence to renewed interest in ADC technology (17). Immunogenicity was overcome by replacing murine antibodies with humanized or fully human antibodies, potency was improved by using drugs that were 100- to 1,000-fold more cytotoxic than previously used drugs, and selectivity was addressed by more careful target and antibody selection. As the result of such improvements, in 2000, gemtuzumab

    ozogamicin (Mylotarg) became the first ADC to be approved

    by the U.S. Food and Drug Administration (FDA) for the treatment of acute myelogenous leukemia (AML). However, this ADC was withdrawn from the market in 2010 because in postmarketing follow-up clinical trials